Ignimbrite

New Zealand geologist Patrick Marshall (1869–1950) coined the term ignimbrite from the Latin igni- [fire] and imbri- [rain].

Ignimbrites are made of a very poorly sorted mixture of volcanic ash (or tuff when lithified) and pumice lapilli, commonly with scattered lithic fragments.

Instantaneously cessation of the flow would cause local compression and extension, which would be evident in the form of tension cracks and small scale thrusting, which is not seen in most ignimbrites.

A model based on observations at the Wall Mountain Tuff at Florissant Fossil Beds National Monument in Colorado suggests that the rheomorphic structures such as foliation and pyroclasts were formed during laminar viscous flow as the density current comes to a halt.

The ash matrix typically contains varying amounts of pea- to cobble-sized rock fragments called lithic inclusions.

Intensely welded ignimbrite may have glassy zones near the base and top, called lower and upper 'vitrophyres', but central parts are microcrystalline ('lithoidal').

The typical range of phenocrysts in ignimbrites are biotite, quartz, sanidine or other alkali feldspar, occasionally hornblende, rarely pyroxene and in the case of phonolite tuffs, the feldspathoid minerals such as nepheline and leucite.

Commonly in most felsic ignimbrites the quartz polymorphs cristobalite and tridymite are usually found within the welded tuffs and breccias.

The chemistry of the ignimbrites, like all felsic rocks, and the resultant mineralogy of phenocryst populations within them, is related mostly to the varying contents of sodium, potassium, calcium, the lesser amounts of iron and magnesium.

Large hot ignimbrites can create some form of hydrothermal activity as they tend to blanket the wet soil and bury watercourses and rivers.

The water from such substrates will exit the ignimbrite blanket in fumaroles, geysers and the like, a process which may take several years, for example after the Novarupta tuff eruption.

In Sierra de Lihuel Calel, La Pampa Province, Argentina, various landforms typical of granites can be observed in ignimbrite.

[15] Ignimbrites occur worldwide associated with many volcanic provinces having high-silica content magma and the resulting explosive eruptions.

The nearby Taupō Volcanic Zone is covered in extensive flat sheets of ignimbrite erupted from caldera volcanoes during the Pleistocene and Holocene.

Successions of ignimbrites make up a large part of post-erosional rocks in Tenerife and Gran Canaria islands.

Yucca Mountain Repository, a U.S. Department of Energy terminal storage facility for spent nuclear reactor and other radioactive waste, is in a deposit of ignimbrite and tuff.

The layering of ignimbrites is used when the stone is worked, as it sometimes splits into convenient slabs, useful for flagstones and in garden edge landscaping.

In the Hunter region of New South Wales, ignimbrite serves as an excellent aggregate or "blue metal" for road surfacing and construction purposes.